The family of edg receptors are commonly grouped with orphan receptors because their endogenous ligands are not known (for example see Hla T and Maciag T (1990) J. Biol. Chem. 265:9308-13 ; U.S. Pat. No. 5,585,476). Recently, however, lysophospatidic acid has been demonstrated to be the endogenous ligand for the edg-2 receptor (Hecht et al. (1996) J. Cell. Biol. 135: 1071-1083; An et al. (1997) Biochem. Biophys. Res. Comm. 213: 619-622).
The edg family of receptors are seven transmembrane G protein coupled receptors (T7Gs). T7Gs are so named because of their seven hydrophobic domains which span the plasma membrane and form a bundle of antiparallel .alpha. helices. These transmembrane segments (TMS) are designated by roman numerals I-VII and account for structural and functional features of the receptor. In most cases, the bundle of helices forms a binding pocket; however, when the binding site must accommodate more bulky molecules, the extracellular N-terminal segment or one or more of the three extracellular loops participate in binding and in subsequent induction of conformational change in intracellular portions of the receptor. The activated receptor, in turn, interacts with an intracellular G-protein complex which mediates further intracellular signaling activities generally the production of second messengers such as cyclic AMP (cAMP), phospholipase C, inositol triphosphate or ion channel proteins.
T7G receptors are expressed and activated during numerous developmental and disease processes. Identification of a novel T7G receptor provides the opportunity to diagnose or intervene in such processes, and the receptor can be used in screening assays to identify physiological or pharmaceutical molecules which trigger, prolong or inhibit its activity.